At present, presensitized plates which utilize positive-working sensitizers containing diazo compounds and phenol resins as main components, or negative-working sensitizers containing acryl series monomers or prepolymers as a main component are put to practical use as lithographic offset printing plates. However, such plates all have low sensitivity, so images are reproduced in these plates through contact exposure using as the printing master, silver salt photographic films in which the images have been recorded in advance. Through advances in computer aided image processing, mass data storage and data communication techniques, and on the other hand, electronic editing systems, wherein input, correction, editing, layout and page allotment of originals are consecutively performed by operating a computer and their resulting copies are taken out in real time as the output of terminal plotters installed in long distant places by utilizing high speed communication network or satellite communication, have been put to practical use in recent years. In particular, in the field of modern printing, in which rapidness is required, has the greatest need for an electronic editing system. In addition, in the field of keeping originals in the form of master films and reproducing printing plates therefrom as occasion arises, it is expected that originals will be stored in recording media in the form of digital data with the development of recording media having very large capacity, such as optical discs.
However, scarcely and direct reproduction system for making printing plates directly from the output of a terminal plotter has been put to practical use as yet. In the present situation, though an electronic editing system is at work, yet the output is recorded in a silver salt photographic film, and the resulting film is superposed on a presensitized plate and subjected to contact exposure, whereby indirectly recording the output in the presensitized plate to make a printing plate. This is because it is difficult to develop direct reproduction type printing plates having sensitivities high enough to make printing plates in a practical time using the output of a plotter as light source (e.g., He-Ne laser, semi-conductor laser).
Under these circumstances, electrophotographic photoreceptors are expected to be useful as photosensitive materials having high sensitivities to provide direct reproduction type printing plates.
As for the printing plate materials (printing masters) utilizing electrophotography, there are known zinc oxide-resin dispersion type offset printing plate materials disclosed, for example, in JP-B-47-47610 (The term "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-48-40002, JP-B-48-18325, JP-B-51-15766 and JP-B-51-25761. In using these materials as printing plates, toner images are formed on the material by the electrophotography, and then the materials are dampened with a desensitizing solution such as an acidic aqueous solution containing a ferrocyanide or a ferricyanide in order to desensitize the non-image area. Although the thus processed offset printing plates have a printing impression typically in the order of from 5,000 to 10,000 sheets, they are unsuitable for printing in which a printing impression higher than the above-described order is required. Moreover, when designed so as to have compositions suitable for desensitization, the plate materials suffer from deterioration of their electrostatic characteristics and produce images of aggravated qualities. Furthermore, the desensitizing solutions used in making the printing plates have the disadvantage of containing harmful cyanides.
In organic photoconductor-resin coated plate materials as disclosed, for example, in JP-B-37-17162 (U.S. Pat. No. 3,139,338), JP-B-38-7758 (U.S. Pat. No. 3,236,640), JP-B-46-39405 (U.S. Pat. No. 3,944,417), electrophotographic photoreceptors of the type which comprise a grained aluminum plate having provided thereon a photoconductive, electrically insulating layer containing, e.g., an oxazole or oxadiazole compound bonded with a styrene-maleic anhydride copolymer are employed. After toner images are formed on those photoreceptors through electrophotography, the non-image area is removed by dissolving it in an alkaline organic solvent to make a printing plate.
On the other hand, the present inventors disclosed electrophotographic presensitized plate materials containing a hydrazone compound and barbituric acid or thiobarbituric acid in JP-A-57-147656 (U.S. Pat. No. 4,500,622) (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). In addition to these plate materials, electrophotographic printing plates sensitized with dyes are disclosed, for example, in JP-A-59-152456, JP-A-59-168462 and JP-A-58-145495. However, these plates are unsatisfactory because the non-image area is adsorbed by ingredients of the electrophotographic photoreceptive layer and becomes contaminated therewith. Also, ink becomes attached to the non-image area of the print, which causes staining of the print, rendering it unusable. To solve this problem, methods for rendering the conductive support hydrophilic by using a physical or chemical means, such as for example, soaking the anodically oxidized surface of an aluminum support in an alkali metal salt of silicic acid, disclosed in U.S. Pat. No. 3,181,461, have been proposed. However, these methods have proved unsatisfactory.
In regard to the so-called lithographic printing plates (presensitized plates), proposals have been disclosed in U.S. Pat. No. 3,860,426, British Patent 2,098,627, JP-B-44-6410 (U.S. Pat. No. 3,634,078), JP-A-60-149191 and JP-A-60-232998 for both the prevention of stain and enhancement of printing durability. However, the image formation in the presensitized plate is achieved by using an o-quinonediazide compound, a diazo compound, a photo-polymerizing system or the like to cause a change in solubility of the light-sensitive layer itself.
Various methods for reducing stains on the electrophotographic printing plates have been proposed. For instance, such methods include the use of casein, polyvinyl alcohol, ethyl cellulose, phenol resin, styrenemaleic anhydride copolymer or polyacrylic acid for the purpose of improvements in adhesiveness and electrophotographic characteristics as disclosed in JP-A-57-147656. Although it is generally said that the hydrophilic strength of the support and the printing durability (adhesiveness between the support and the photoreceptive layer) have a reciprocal relationship to each other, JP-A-59-45458 discloses the use of definite amounts of polyacrylic acid as an interlayer for the purpose of simultaneous improvement in both the aforesaid properties. In addition, JP-A-56-19063 discloses a method of coating a photoconductive pigment with a resin for the purpose of prevention of staining. However, effects brought about by such methods have been found to be unsatisfactory.